Liquid fuel production via supercritical water gasification of algae: a role for solar heat integration?

Algal biomass is an attractive feedstock for carbon-neutral fuel production due to high growth rates and its potential to be farmed in artificial ponds on non-arable land. Supercritical water gasification (SCWG) of algae is appealing because it eliminates a drying step and offers fast kinetics, low char/tar rates, high carbon efficiency and good scale-up potential. Limited understanding of the algae breakdown mechanisms in SCWG, including kinetics and char formation, together with high pressures and temperatures, challenging for containment materials, make reactor design difficult. Dewatering and pumping of algae at high concentration in a cost-effective manner is also challenging. Integration of concentrated solar-thermal (CST) heat into SCWG of algae offers the potential for lower-cost fuel production, since CST is a cheaper energy source than unprocessed algae. However, CST heat integration is challenging due to the additional impact of sunlight variability. Integration of intermittent solar synthesis gas flows with downstream liquid-fuel processing such as the Fischer–Tropsch or methanol synthesis raises other challenges. Off-sun SCWG reactors downstream of a high-temperature salt storage offer future potential. The falling cost of green hydrogen input offers new opportunities to achieve lower-cost solar-SCWG system configurations. This review provides a general overview of algae-fed SCWG technology and challenges, then discusses CST-integrated SCWG of algae and its prospects for lower-cost liquid fuel production.